What caused the brief false Spamhaus listing in October 2020, and what does it reveal about blocklist QA processes?
Michael Ko
Co-founder & CEO, Suped
Published 3 Jul 2025
Updated 15 Aug 2025
7 min read
Email deliverability relies heavily on the accuracy of DNS-based blocklists, often referred to as DNSBLs. These systems are designed to identify and flag IP addresses or domains associated with spam and malicious activity, preventing unwanted emails from reaching inboxes. However, even the most reputable blocklists can experience hiccups, leading to temporary false positives that can disrupt legitimate email flows.
One such event occurred in October 2020, involving Spamhaus, a widely respected provider of blocklists. This brief false listing, though quickly resolved, offered a rare glimpse into the complex quality assurance processes that underpin these critical services. It highlighted the challenges blocklist operators face in maintaining high accuracy while rapidly responding to evolving threats.
For email senders, understanding what happened during this particular incident, and what it tells us about blocklist operations, can provide valuable insights. It underscores the importance of robust monitoring and quick response strategies for maintaining email deliverability.
A false positive occurs when a legitimate IP address or domain is erroneously added to a blocklist (or blacklist). Instead of blocking only spam, the system mistakenly flags valid senders, leading to delivery failures for their emails. This can be incredibly frustrating for businesses and individuals who rely on email for communication, as their messages may be unfairly routed to spam folders or rejected outright.
While blocklists are essential tools for combating spam and enhancing email security, false positives can undermine their effectiveness and create significant headaches for senders. The impact can range from temporary delays to substantial financial losses, especially for companies that rely on email for critical operations like transactional messages or marketing campaigns.
Such incidents, especially system-wide ones, are relatively rare for well-established blocklist providers like Spamhaus. Their systems are generally highly refined to minimize errors while maximizing detection of malicious activity. Nevertheless, these rare occurrences serve as important reminders that no system is foolproof and that continuous vigilance is required from both blocklist operators and email senders.
Dissecting the October 2020 Spamhaus incident
In October 2020, reports emerged of a brief but noticeable false listing by Spamhaus, affecting a small subset of IP addresses. Initial queries in online communities, such as the sysadmin subreddit, indicated that some users observed unexpected listings, though many others saw no issues on their monitors. This discrepancy suggested a targeted or rapidly corrected problem, rather than a widespread, lingering outage.
The core issue was later attributed to an internal problem at Spamhaus itself. A new detection mechanism or rule, which had successfully passed internal testing, produced false positives once deployed in a live production environment. The listings were active for a very short duration, estimated to be less than 20 to 30 minutes, before being quickly pulled. This rapid identification and rollback prevented a more significant impact across the internet.
The incident was an example of how even with rigorous pre-production testing, real-world conditions can expose unexpected behaviors in complex systems like those used by blocklist providers. For further context, you can read more about Spamhaus's false positive issue from a related incident. This highlights the constant battle against spam while striving for accuracy.
Insights into blocklist QA processes
This incident, though minor in its overall impact due to the quick response, offers crucial insights into the quality assurance (QA) processes of major blocklists. It demonstrates that even the most advanced systems can occasionally encounter unforeseen issues once they interact with the dynamic and unpredictable nature of live email traffic.
The rapid detection and rollback of the erroneous listings point to a well-oiled QA and incident response mechanism. This suggests that leading blocklists (or blacklists) employ sophisticated monitoring systems that can quickly identify anomalies in their data and distribution, allowing for immediate corrective action. Such a process is vital for maintaining the trust and reliability of their services, as prolonged false positives could severely damage their reputation.
Effective QA for blocklists goes beyond initial testing; it involves continuous monitoring, real-time feedback loops, and a robust system for emergency mitigation. Here's a look at key aspects of a strong blocklist QA process:
Aspect
Description
Implication for senders
Real-time monitoring
Spamhaus and others constantly watch for anomalous listing patterns and immediate impacts.
Reduces duration of any false listings.
Automated rollback systems
Ability to quickly revert to previous stable versions of data or rulesets.
Minimizes downtime and email disruption.
Diverse testing environments
Testing new rules against a wide range of real and simulated traffic before production deployment.
Reduces the likelihood of false positives reaching production.
Community feedback integration
Systems for receiving and quickly acting on reports from affected users.
Provides an additional layer of real-world validation and early warning.
Mitigating the impact of false blocklist listings
Even with robust QA processes, incidents like the one in October 2020 can happen. For email senders, the key is not to prevent these rare occurrences, but to be prepared to mitigate their impact swiftly. Proactive measures and a clear response plan are crucial for minimizing disruption to your email deliverability.
One of the most important steps is continuous blocklist monitoring. Being aware of your IP and domain status across major blocklists allows for immediate detection of any listing, whether legitimate or a false positive. This quick notification enables you to initiate a response before significant email delivery issues arise.
Beyond monitoring, maintaining excellent email sending hygiene is your best defense. This includes practices such as regularly cleaning your mailing lists, avoiding sending to unengaged recipients, and ensuring proper email authentication protocols like SPF, DKIM, and DMARC are correctly implemented. Good hygiene significantly reduces your risk of legitimate blocklist inclusion, making it easier to distinguish between true threats and anomalies.
If you find your IP or domain listed, even briefly, it's vital to have a clear strategy for delisting and communication. Understanding how to get delisted from Spamhaus blacklists is key. While false positives are typically resolved quickly by the blocklist operator, having a plan to manually request removal or contact support if the issue persists can save valuable time and reputation.
Proactive approach
Continuous vigilance: Employ automated tools for real-time monitoring of your sending IPs and domains against major email blocklists.
Maintain hygiene: Regularly audit your sending practices to prevent legitimate listings from spam traps or user complaints. Learn more about Spamhaus listings triggered by spam traps.
Reactive approach
Rapid response plan: Have a predefined process for identifying the cause of a listing and initiating delisting requests. Understand what to do if listed in Spamhaus.
Communicate swiftly: Inform stakeholders (internal teams, customers) about potential email delivery issues and the steps being taken.
Views from the trenches
Best practices
Implement automated blocklist monitoring to detect listings immediately and reduce impact.
Maintain pristine email list hygiene to prevent legitimate blacklisting, making false positives easier to spot.
Regularly review your email authentication (SPF, DKIM, DMARC) to ensure strong sender reputation.
Common pitfalls
Not having a clear action plan for when an IP or domain gets listed, leading to delayed resolution.
Ignoring minor or temporary listings, which can indicate underlying deliverability issues.
Failing to communicate internally or externally when email delivery is affected by a blocklist event.
Expert tips
Set up alerts for all major blocklists, not just the most common ones.
Understand the typical delisting process for each blocklist you rely on.
Segment your email traffic to isolate potential issues quickly if one part of your sending infrastructure is affected.
Marketer view
Marketer from Email Geeks says they did not see anything on their monitors for their network, suggesting the false positive was localized or very short-lived.
October 30, 2020 - Email Geeks
Expert view
Expert from Email Geeks confirms that the listings were active for a very short period, specifically less than 20-30 minutes.
October 31, 2020 - Email Geeks
Lessons learned and moving forward
The brief false Spamhaus listing in October 2020 served as a valuable case study, demonstrating that while blocklists are robust, they are not immune to errors. It underscored the critical role of sophisticated QA processes and rapid response mechanisms in maintaining the integrity of these systems. For email senders, the takeaway is clear: proactive monitoring and strong sending hygiene are paramount. These practices ensure you are prepared for any unexpected deliverability challenges, whether caused by a legitimate blocklist event or a rare false positive.
By understanding the nuances of how blocklists operate and what to do when issues arise, you can safeguard your email deliverability and ensure your messages consistently reach their intended recipients.
Spamhaus
0Spam
Cisco
NoSolicitado
URIBL
abuse.ro
ALPHANET
Anonmails
Ascams
BLOCKEDSERVERS
Calivent Networks
EFnet
JustSpam
Kempt.net
NordSpam
RV-SOFT Technology
Scientific Spam
Spamikaze
SpamRATS
SPFBL
Suomispam
System 5 Hosting
Team Cymru
Validity
www.blocklist.de Fail2Ban-Reporting Service
ZapBL
2stepback.dk
Fayntic Services
ORB UK
technoirc.org
TechTheft
Spamhaus and others constantly watch for anomalous listing patterns and immediate impacts.
